1. Field of the Invention
The present disclosure relates to a method and apparatus of operating a transistor.
2. Description of the Related Art
The Insulated Gate Bipolar Transistor (IGBT) has found recent application in power modules in electric drives in electric, fuel cell, gasoline/electric hybrid and fuel cell/electric hybrid vehicles, amongst other areas. One aspect of critical importance in electric drives in such applications is the power density. Therefore, electric drives are often designed with the highest power density possible. IGBT chips used in these modules are often, therefore, operated under extreme conditions including high junction temperature, very fast switching speeds and high voltage and current ratings. With IGBTs operated at such extreme conditions, the fault response time with traditional IGBT fault protection approaches, for example, Vce sensing, is not fast enough.
Under switching operation known in the art, the IGBT gate is driven by a pair of power transistors where the bases of the power transistors are controlled with a local controller. A typical output stage of a conventional IGBT gate driven circuit configuration is illustrated in FIG. 1.
Under a fault condition, such as a shoot through fault, the potential between the collector and the emitter of the IGBT (Vce) will be high. The high potential Vce during switching ON triggers a desaturation fault condition in the local controller. In response, the local controller softly turns OFF the IGBT and sends a fault signal to a system controller.
Current third-generation IGBTs have extremely high current ratings (e.g., up to 1000 A), low saturation voltages (e.g., one to two volts), and very fast switching times (e.g., less then a few hundred nanoseconds). Therefore, use of conventional gate drive circuitry with third generation IGBTs creates problems including poor response time upon fault, poor control of shoot through current and poor control of voltage overshoot at turn off.
For example, with conventional IGBT fault control approaches, the fault signal is sensed through the local collector of the IGBT, where the sensed voltage is compared with a preset voltage. If the sensed voltage is higher then this preset voltage, a timer starts which is usually done with a capacitor charging mechanism. Common practice is to set the timer at a few microseconds. When the time is up, the local controller softly shuts the IGBT OFF. Recently developed third generation IGBTs with high current ratings and high switching speeds cannot wait a few microseconds once a fault occurs; the protection must be activated within a few hundred nanoseconds.
IGBT fault protection approaches recently described involve Vce sensing through a desaturation diode. There are two main issues associated with this approach: delayed fault detection due to blanking time required for noise rejection; and lack of dynamic feedback information provided (e.g., only logic two states exist: fault or not fault).
Since the Vce signal is extremely noisy, blanking time is usually used to reject the noise from normal switching transient. For high performance power modules, dynamic feedback information is desirable to control the fault state effectively. To compensate for the lack of dynamic feedback information associated with Vce sensing, conventional approaches make use of a second feedback loop to monitor the collector current. A different fault detection approach that does not employ Vce sensing has been disclosed where gate voltage is the only parameter sensed and controlled. This approach may not be suitable for high performance IGBT modules where fast detection and dynamic control is required. Existing IGBT fault protection approaches that sense a change in current with respect to time (di/dt) are limited to sensing di/dt by measuring the voltage across the stray inductance where the measured voltage is then compared with a threshold. However, the outcome is still a logic state: fault or not fault; hence no dynamic control is involved.
For applications involving high performance IGBTs, dynamic fault information may be particularly beneficial. Therefore, there remains a need in the art for an approach to IGBT control that addresses these issues. The present disclosure addresses these needs and provides associated benefits.